Demand of high tensile strength steel sheets has been increased in order to decrease the body weight for improving the fuel cost of automobiles. These steel sheets must satisfy the following various requirements.
1. Non-ageing. PA0 2. r value is high. PA0 3. Elongation is high. PA0 4. Yield ratio is low. PA0 5. Inclusions are few and there is no surface defect. PA0 6. Production cost is not high. PA0 7. Hot-dip galvanization is easily carried out. PA0 (1) When the steel having Al content which is at least 4 times of N amount and is at least 0.02%, is hot rolled at a reduction rate of at least 90%, a rolling speed of at least 40 m/min and a coiling-up temperature of 600.degree.-750.degree. C., if an amount of Nb added with respect to C is about at least 1/3 of the equivalent as NbC, the cold rolled and annealed product shows the non-ageing property. PA0 (2) The steel in which the solid solute Nb not combined with C is less than 0.020%, is larger in the elongation and is equal in r value as compared with the steel containing the larger amount of Nb. PA0 (3) When to the steel in which an amount of Nb is about at least 1/3 of the equivalent based on C and the uncombined Nb is not more than 0.020%, are alone added P, Si and Mn respectively, the decrease of r value is most remarkable in Mn and the decrease of r value due to the addition of Si is next and the influence of P is smallest. PA0 (4) When the extremely low carbon steel added with at least 0.05% of P is subjected to batch annealing, the cold work embrittlement after press working is caused but when the continuous annealing is effected at a cooling rate of at least 1.degree. C./sec, even if P of not more than 0.10% is contained, the cold work embrittlement is not caused. PA0 (5) When at least 0.03% of P is contained, if not more than 0.8% of Mn or not more than 1.2% of Si is contained, the deterioration of r value is low and the high tensile strength is obtained. PA0 (6) The hot-dip galvanizing property of the steel sheets containing Nb, P and Mn within the above described composition range is satisfied when the Si content is not more than 0.5%.
As the steels satisfying these requirements, dual phase steel composed of martensite-ferrite two phase alloy structure and rephos-steel in which P, Mn, Si and the like are added to aluminium killed steel have been developed but these steels are poor in the formability and therefore cannot be used in the portions where the deep drawing is carried out, such as automobile fenders. Furthermore, these steels contain a number of elements, such as Si, Cr, Mn and the like which retard the plating ability, so that the hot-dip galvanization cannot be easily effected.
Furthermore, it has been proposed that Ti or Nb having a high affinity to C and N is incorporated in an amount of more than stoichiometric equivalent based on an amount of C or C+N to fix C and N and Mn or Si is incorporated as a solid solution reinforcing element to obtain a steel sheet having high r value and tensile strength. For example, it has been reported in "Iron and Steel", 1979, No. 11, p. 838 that an extremely low carbon steel having less than 0.01% of C which contains 0.25% of Ti, 1.5% of Mn and 0.22% of Si is continuously annealed to obtain a steel having high formability which has a tensile strength (abbreviated as TS hereinafter) of 43 kg/mm.sup.2, an yield point (abbreviated as YP hereinafter) of 22-25 kg/mm.sup.2, r value of 1.8 and an elongation of 39%. However, this steel has a demerit that Ti combines with not only C and N in the steel but also S and O, so that a large amount of inclusions are formed and the surface defects are liable to be formed. Moreover, amounts of Mn, Si and Ti added are large and it is necessary to add a large amount of alloy to effect decarburization refining to decrease c in the molten steel to less than 0.01%, so that this steel has a demerit that the cost of alloy and decarburization is higher.
Furthermore, it has been proposed in Japanese Patent Laid-Open Application No. 100,920/79 that high tensile strength steels having TS of 42-46 kg/mm.sup.2, YP of 28-30 kg/mm.sup.2, r value of 1.6-1.8 and an elongation of 32-35% are obtained by subjecting a steel containing 0.004% of C, 1.01% of Si, 0.22% of Mn, 0.025% of Al and 0.049% of Nb to a continuous annealing and then to an over ageing treatment at 400.degree. C. for 3 minutes, but r value and the elongation are low, so that the formability is not satisfactory.